fix bug in not properly updating squares vector

fcore/quad-tree.sml

@@ -226,14 +226,9 @@ struct
         end
 
   fun insertItemIntoTree (item, acc) =
-    if #data item = 0 then
-      (* ignore specific data by not inserting it into tree.
-       * May later functorise this quad tree,
-       * and allow different types of data
-       * to be ignored/stored in #data field. *)
-      acc
-    else
+    if #data item <> 0 then
       BinTree.insert (item, acc)
+    else acc
 
   fun toList qtree =
     let val tree = foldWithDuplicates (insertItemIntoTree, qtree, BinTree.empty)
@@ -255,7 +250,9 @@ struct
             val startY = Ndc.fromPixelY (y, windowWidth, windowHeight)
             val endY = Ndc.fromPixelY (ey, windowWidth, windowHeight)
 
-            val acc = Ndc.ltrbToVertex (startX, startY, endX, endY) :: acc
+            val vec = 
+              Ndc.ltrbToVertexRgb (startX, startY, endX, endY, 0.0, 0.0, 0.0)
+            val acc = vec :: acc
           in
             loop (windowWidth, windowHeight, tl, acc)
           end
@@ -265,6 +262,8 @@ struct
       let
         val qtree = build (0, 0, size, squares)
         val squares = toList qtree
+        val msg = List.length squares
+        val () = print (Int.toString msg ^ "\n")
       in
         loop (windowWidth, windowHeight, squares, [])
       end